The present invention relates generally to electronic keyboard musical instruments and, more particularly, to a circuit for developing alternately repeating keying signals of the type typifying the playing of, for example, a marimba.
A marimba is a percussive musical instrument normally played by alternately striking two groups of one or more bars of the instrument to produce a repetitive musical pattern. Attempts have heretofore been made to simulate this effect in an electronic keyboard musical instrument, e.g. an electronic organ, wherein a pair of notes are repetitively caused to sound in an alternating manner by depressing or holding down the keys corresponding to the two notes. The effect may also be simulated when more than two keys are depressed by grouping the depressed keys into two groups, all of the notes within each group being sounded simultaneously.
Prior art circuits adapted for simulating the foregoing alternating repeat effect typically incorporate an alternating bus scheme wherein each key of the keyboard is coupled to one of a pair of buses, the two buses being alternately energized by a suitable oscillator. As a result, depressing at least one key coupled to each of the buses causes the corresponding notes to alternately sound on a repetitive basis. A major problem with this type of circuit is that there are certain combinations of notes which do not alternate when the corresponding keys are depressed or that alternate in a very undesirable manner. For example, if two keys are depressed both of which are coupled to the same bus, the corresponding notes will not be alternately sounded but, rather, will be simultaneously sounded in a repetitive manner. Also, if a plurality of keys are depressed, a large number of which are coupled to one bus while a smaller number are coupled to the second bus, the alternately sounding signals will be composed of correspondingly different numbers of notes creating an undesirable musical effect.
While the foregoing problems are relatively serious in a conventional electronic organ where each key is associated with its own individual keyer, the problems are further compounded in a computer or microprocessor based organ. In organs of the latter type only a limited number of tone generator sources are normally provided, each of the tone generator sources being programmable and assignable together with one of a limited number of keyers for producing the tone signal corresponding to any key of the organ. Since it is therefore not known beforehand which of the tone generators and keyers will be assigned to any particular key, the prior art alternating bus scheme is particularly unsuited for use in this type of an organ.